The present invention relates to a device for clamping threaded oil well and water well tubulars and rods, including but not limited to drill pipe, drill collars, well casing, production tubing, sucker rods, pump column pipe, and the like, all of which threaded tubulars and rods are hereinbelow referred to simply as "down-hole tubulars". More particularly, this invention relates to such a clamp which precisely centers down-hole tubulars of varying diameters.
In well drilling and well completion operations it is necessary to lift and precisely align lengths of down-hole tubulars. For example, in oil or water well drilling, multiple lengths of drill pipe must often be raised from a horizontal position at or near ground level to a vertical position aligned with the centerline of the well. Such lifting and aligning operations require a clamp for securely holding the pipe in place as it is lifted. When a pivotably mounted pipe boom is used, this clamp must support large loads in several different orientations.
Compounding the problem is the fact that each joint or length of a down-hole tubular must be closely aligned with a string of such tubulars after it has been lifted to the vertical position, as when a string of drill pipe or casing is being made up, for example. A clamp for this purpose should preferably provide the necessary alignment for down-hole tubulars having various diameters, without any adjustment. Proper alignment has been a problem for many such clamps of the prior art, especially those employing pivoted clamping jaws.
When pivoted clamping jaws are used, there is a tendency for the center of the clamped down-hole tubular to vary as a function of the diameter of the tubular being clamped. This problem may be alleviated somewhat by using guided jaws in conjunction with symmetrically moving pivoted rocker arms. Such an arrangement is shown in a machine tool clamp described by Lorenz in U.S. Pat. No. 3,386,726. In the Lorentz clamp the guided jaws are free to translate with respect to the pivotably mounted rocker arms as the clamp closes.
This approach, however, suffers from the disadvantage that loads are not symmetrically distributed in the clamp for the full range of clamp positions. As the guided jaw translates with respect to the rocker arm, the center of clamping force on the jaw moves.